During the early 1990s, Sandia National Laboratory developed a single-chip hydrogen sensor that utilized Palladium-Nickel (PdNi) metal films as hydrogen gas sensors. U.S. Pat. No. 5,279,795, naming Robert C. Hughes and W. Kent Schubert as inventors, assigned to the United States as represented by the U.S. Department of Energy, describes such a sensor and is incorporated by reference herein.
One of the key benefits of the sensor described in the '795 patent is its ability to detect a dynamic range of hydrogen concentrations over at least six orders of magnitude. Prior solutions to the problem of detecting hydrogen concentrations had been generally limited to detecting low concentrations of hydrogen. These solutions include such technologies as metal-insulator-semiconductor (MIS) or metal-oxide-semiconductor (MOS) capacitors and field-effect-transistors (FET), as well as palladium-gated diodes.
The hydrogen sensor described in the '795 patent was a notable advance in hydrogen-sensing technology. It was, however, primarily limited to an experimental laboratory environment due to the difficulties encountered in manufacturing such a sensor.
In typical silicon fabrication facilities, metal films are first blanket-deposited across the entire wafer, and are subsequently patterned by an etch process. However, conventional etchants for PdNi also attack aluminum, which is normally present on the wafer surface as an interconnect metal before the PdNi film is deposited. Patterning the PDNI by etching would also attack the unprotected aluminum, destroying the sensor. Other non-conventional semiconductor fabrication techniques involving the use of a photoresistive material applied before the PdNi in a “lift-off” process have produced very low yields in tests performed by the assignee of the present invention. Low yields in the production of semiconductor devices typically translates to difficulties in producing a commercializable product.
One solution to the above problems is described in U.S. patent application Ser. No. 09/729,147, titled “Robust Single-Chip Hydrogen Sensor,” assigned to Honeywell International Inc., and incorporated by reference herein. The technique disclosed is a lift-off process, in which an adhesion promoting layer, such as chromium, is provided to cause a sense-resistive layer, such as a PdNi layer, to adhere to an underlying base layer. As a result, during the lift-off process, there is a reduced likelihood of sensor portions being lifted off in conjunction with the portions actually intended to be removed. However, the use of chromium has been discovered to be prone to at least one disadvantage. The chromium has a tendency to affect the operation of hydrogen sensing transistors on the sensing chip. As a result, accuracy and/or sensing range may be affected.
It would thus be desirable to provide a robust single-chip hydrogen sensor that is capable of sensing hydrogen concentrations over a broad range, such as from approximately 1% to approximately 100% concentrations.
It would also be desirable for such a sensor to be efficiently manufacturable, so that costs are reduced and the sensor is producible in high enough yields to enable commercialization.
It would be desirable for such a sensor to provide measurement results that approximate or improve on the results from previous hydrogen sensors.
It would additionally be desirable to minimize sensor drift and to improve device-to-device and wafer-to-wafer repeatability.